The present invention relates to a method and apparatus for maintaining an incident optical signal having a low degree of polarization in a specific state of polarization at a terminal of an optical waveguide.
In the technical field of transmitting a light beam while maintaining its state of polarization, a special optical fiber capable of maintaining the state of polarization, i.e., a polarization maintaining optical fiber is often used. xe2x80x9cIntroduction to Optical Fiber Communicationsxe2x80x9d (Yasuharu Suematsu et.al., OHM-sha, Ltd., pp. 197-199, Mar. 10, 1991, Third Edition) describes polarization maintaining optical fibers. An optical fiber having increased elliptical deformation and axially asymmetrical side pits in the gradient index to increase structural axial asymmetry, an elliptical jacket fiber having a double cladding structure and an elliptically deformed intermediate cladding to apply stress to the core, and the like are available in addition to the PANDA fiber (Polarization maintaining and absorption reducing fiber) having a structure in which stress applying portions are formed in the cladding to apply anisotropic stress to the core.
The most popular PANDA fiber has only two directions of the plane of polarization allowed to be maintained, as shown in FIG. 16. The directions of the plane of polarization of the remaining polarization maintaining optical fibers described above are also limited to specific directions.
As shown in FIG. 16, reference numeral 101 denotes a cross-section of the PANDA fiber. Stress applying portions 103a and 103b are present on the cross-section 101 so as to sandwich a core 102 through which a light beam passes. Directions 104 and 105 along which the polarization maintaining optical fiber can maintain the plane of polarization are a direction connecting the centers of the stress applying portions 103a and 103b, and a direction perpendicular to the above direction, as indicated by chain lines. No incident light beam having the plane of polarization in a direction except the above two directions can be maintained.
To solve this problem in a conventional arrangement, as shown in FIG. 17, a light source 106a having a high degree of polarization like a semiconductor laser is used. A polarization controller 108 serving as a means for arbitrarily changing the plane of polarization is arranged in front of a polarization maintaining optical fiber 107. The plane of polarization of a laser beam must be aligned in a polarization maintainable direction of the polarization maintaining optical fiber 107, and the aligned laser beam must be guided to the polarization maintaining optical fiber 107.
Referring to FIG. 17, reference numeral 109 denotes a single-mode optical fiber for connecting the light source 106a and the polarization maintaining optical fiber 107; and 110, a connecting portion between the polarization maintaining optical fiber 107 and the single-mode optical fiber 109. The connecting portion comprises an optical connector or may be fusion-spliced. Reference numeral 111 denotes an output terminal of the polarization maintaining optical fiber 107. The polarization controller 108 is located midway along the single-mode optical fiber 109 in FIG. 17. However, the polarization controller 108 may be arranged between the single-mode optical fiber 109 and the polarization maintaining optical fiber 107.
FIG. 18 shows the state of a propagation light beam in the optical circuit system shown in FIG. 17. Assume that a light beam emitted by the light source 106a having a high degree of polarization is an elliptically polarized light beam 112a (reference numeral 113 denotes its plane of polarization) (linear polarization and circular polarization correspond to special cases of elliptical polarization). In this case, the polarization controller 108 is inserted midway along the single-mode optical fiber 109 to convert the elliptically polarized light beam 112a into a linearly polarized light beam 114 parallel to a polarization maintaining plane 104 of the polarization maintaining optical fiber 107, and the linearly polarized light beam 114 is incident. The state of polarization in the polarization maintaining optical fiber 107 and the state of polarization of an exit light beam from the output terminal 111 can be kept constant.
FIG. 19 shows the state of a propagation light beam when the polarized light beam 112a emitted by the light source 106a having a high degree of polarization is incident on the polarization maintaining optical fiber 107 such that the plane 113 of polarization of the polarized light beam 112a is entirely different from the polarization maintaining direction of the optical fiber. That is, this is equivalent to the case in which the polarization controller 108 is omitted from the arrangement in FIG. 18. In this case, the polarization maintaining optical fiber 107 has no longer the polarization maintaining function. A polarized light beam 112b different from the polarized light beam 112a propagates through the polarization maintaining optical fiber 107 and emerges from the output terminal 111 in a state 115 of polarization different from the plane 113 of polarization. When the light beam receives perturbation 116 at any position in the polarization maintaining optical fiber 107, the state of polarization of the exit light beam changes from 115 to 117 in accordance with the perturbation 116. The perturbation is disturbance such as a dynamic stress, stress based on temperature or humidity, or the like.
Consider an optical system for allowing a polarization analyzer 118 to observe a light beam controlled by a polarization controller 108, as shown in FIG. 20. This system is obtained by adding the polarization analyzer 118 to the output terminal 111 in FIG. 17. The observation result is expressed on the Poincarxc3xa9 sphere using a state of polarization as the Stokes parameter, as shown in FIG. 21. Reference numeral 1101 denotes clockwise circular polarization; 1102, xe2x88x9245xc2x0 linear polarization; 1103, vertical linear polarization; 1104, +45xc2x0 linear polarization; 1105, horizontal linear polarization; and 1106, counterclockwise circular polarization. When the polarization controller 108 aligns the plane of polarization of the elliptically polarized light beam 112a with the plane of polarization of the polarization maintaining optical fiber 107, the plane of polarization observed by the polarization analyzer 118 is observed on an equator 1107 of the Poincarxc3xa9 sphere. That is, the observed light beam is any one of the linearly polarized light beams. When the plane of polarization of the elliptically polarized light beam 112a is not aligned with the plane of polarization of the polarization maintaining optical fiber 107, the state on the equator 1107 of the Poincarxc3xa9 sphere cannot be maintained upon receiving the perturbation in an unstable state of polarization. The state of polarization irregularly changes, as indicated by reference numeral 1108.
As described above, in light beam propagation using a light source having a high degree of polarization like a semiconductor laser while maintaining a state of polarization, the plane of polarization of an exit light beam from the semiconductor laser must be converted into the polarization maintaining direction of the polarization maintaining optical fiber using a means (e.g., a polarization controller) capable of arbitrarily changing the state of polarization. The converted light beam must be guided to the polarization maintaining optical fiber. The means for aligning the state of polarization with the polarization maintaining plane of the polarization maintaining optical fiber is generally expensive and complex. It is also possible to incorporate the polarization maintaining optical fiber as a semiconductor laser module while aligned with the plane of polarization of the exist light beam from the laser in advance. This semiconductor laser module is expensive. Much care must be taken for use of the semiconductor laser incorporating this semiconductor laser module in a measuring device requiring high precision.
An OTDR (Optical Time Domain Reflectometry) method has been used as an effective method of measuring the light beam return loss or the like of an optical waveguide or optical component. According to this method, an optical pulse is incident on a target measurement optical fiber, a Fresnel reflected light beam from the terminal or connecting portion of an optical fiber or a backscattering light beam from the interior of an optical fiber is received. The propagation loss in the optical fiber or the connection loss at the connecting point is measured from the reception intensity of the reflected light beam as a function of arrival time, i.e., the return loss distribution. In the general OTDR method, the spatial resolution and measurement sensitivity are limited to several 10 cm and about 60 dB, respectively, due to the influence of Rayleigh scattering.
To improve the measurement sensitivity and spatial resolution, an OLCR (Optical Low Coherence Reflectometry) method is proposed. This method is obtained by applying a Michelson optical interference circuit to the OTDR method.
Prior to a description of the OLCR method, a Michelson optical circuit made up of optical fibers will b e described below. The basic arrangement of the Michelson optical circuit is shown in FIG. 22. Referring to FIG. 22, reference numeral 1201 denotes a light source; 1202, an optical detector; 1205, an optical system including a collimator lens 1203 and a movable reflecting mirror 1204; 1207, a terminal connecting a test sample 1206; 1208, an optical multiplexer/demultiplexer; 1209, a first optical waveguide connecting the light source 1201 and the optical multiplexer/demultiplexer 1208; 1210, a second optical waveguide connecting the optical detector 1202 and the optical multiplexer/demultiplexer 1208; 1211, a third optical waveguide connecting the optical system 1205 and the optical multiplexer/demultiplexer 1208; 1212, an output terminal for a light beam from the third optical waveguide 1211 to the optical system 1205; and 1213, a fourth optical waveguide connecting the optical multiplexer/demultiplexer 1208 and the terminal 1207 connecting the test sample. The first optical waveguide 1209 comprises a first portion 1209a on the light source side and a second portion 1209b on the optical multiplexer/demultiplexer side. A polarization controller 1214 is inserted midway along the first portion 1209a. Reference numeral 1215 denotes a connecting portion between the first and second portions 1209a and 1209b of the first optical waveguide 1209.
The optical waveguides in this Michelson optical interference circuit comprise optical fibers and are required to suppress level variations in interference signal by variations in polarization of a propagation light beam. The state of polarization of a light beam propagating through the optical fiber must be maintained. For this purpose, polarization maintaining optical fibers are used as the optical waveguides. To guide a polarized light beam into the polarization maintaining optical fiber, a semiconductor laser having a high degree of polarization is generally used as a light source. The polarization controller is also used to align the plane of polarization of a light beam emitted by the light source with the polarization maintaining plane of the polarization maintaining optical fiber.
In FIG. 22, a semiconductor laser is used as the light source 1201. A single-mode optical fiber is used as the first portion 1209a of the first optical waveguide 1209. Polarization maintaining optical fibers are used as the second portion 1209b of the first optical waveguide 1209, the second optical waveguide 1210, the third optical waveguide 1211, and the fourth optical waveguide 1213, respectively. The polarization controller 1214 adjusts the plane of polarization of a light beam emitted by the light source 1201 to align with the polarization maintaining plane of the polarization maintaining optical fiber (second portion of the first optical waveguide) 1209b. If no adjustment is made, the state of polarization of a light beam propagating through the polarization maintaining optical fiber of the optical interference circuit becomes unstable, as described with reference to FIGS. 19 and 21.
A light beam emitted by the light source 1201 and having the adjusted plane of polarization passes through the first optical waveguide 1209 and is demultiplexed by the optical multiplexer/demultiplexer 1208 into two light beams. One light beam passes through the fourth optical waveguide 1213 and is guided to the test sample 1206. This light beam is reflected by the connecting terminal 1207 of the test sample 1206 or the interior of the test sample 1206. The reflected light beam returns to the optical multiplexer/demultiplexer 1208 through a reverse path. The other light beam passes through the third optical waveguide 1211, the connecting portion 1212, and the collimator lens 1203, and is reflected by the reflecting mirror 1204. The reflected light beam returns to the optical multiplexer/demultiplexer 1208 through a reverse path. The two reflected light beams returning to the optical multiplexer/demultiplexer 1208 cause interference when the optical path length from demultiplexing to reflection of one light beam is equal to that of the other light beam. The interference light beam is demultiplexed by the optical multiplexer/demultiplexer 1208 again, and the two light beams are guided to the optical detector 1202. An interference light intensity corresponding to the reflectances of the two light beams is sent to the optical detector 1202.
The reflecting mirror 1204 is movable. When the distance between the collimator lens 1203 and the reflecting mirror 1204 is changed, an interference signal between a light beam from the light source and a wave reflected by a defect or the like in the test sample 1206 at a position corresponding to the optical path light equal to that of the light beam from the light source can be detected by the optical detector 1202. The interference signals as a function of the positions of the reflecting mirror 1204, i.e., light beam return losses are plotted to obtain the distribution of defect positions in the optical fiber.
To cause stable interference in the Michelson interference optical system shown in FIG. 22, the state of polarization of a light beam must be constant at the input position (connecting terminal) 1207 to the test sample 1206 and the input position (output terminal) 1212 to the optical system 1205. That is, the optical waveguides 1211 and 1213 must have the function of maintaining the state of polarization.
The arrangement of a light beam return loss measuring device using the Michelson optical circuit shown in FIG. 22 is shown in FIG. 23. This arrangement is obtained by adding an arithmetic means 1216 and a display means 1217 to the Michelson optical circuit in FIG. 22. The arithmetic means 1216 calculates the return loss of the test sample on the basis of an output from the optical detector 1202. The display means 1217 displays the return loss of the test sample measured by the arithmetic means 1216.
The OLCR method is described in Applied Optics Vol. 26, No. 9, xe2x80x9cNew measurement system for fault location in optical waveguide devices based on an interferometric techniquexe2x80x9d, Kazumasa Takada, et.al., November, 1989. The Michelson optical interference circuit is applied to the OTDR method to greatly improve the measurement sensitivity and spatial resolution.
The arrangement of the OLCR method in the above reference is shown in FIG. 24. This arrangement is based on the arrangement shown in FIG. 22. Only differences will be described. First, a single-mode optical fiber is not used as a first portion 1209 of a first optical waveguide 1209, and a light beam emitted by a light source 1201 passes in the air and is incident on a polarization controller 1214. The light beam then passes in the air and is incident on a second portion 1209b of the first optical waveguide. This difference s not essential. Second, a superluminescent diode (SLD) having a high degree of polarization but a low coherent radiation wave is used as the light source 1201. This diode has a wide emission spectral range having a center frequency of 1.3 xcexcm and a half-width of 0.04 xcexcm and allows high-output continuous oscillation. The wide range facilitates adjustment to a condition for causing interference using the frequency as the parameter. Third, a phase modulator 1218 is inserted midway along a fourth optical waveguide 1213 connecting a terminal 1207 connecting the test sample and an optical multiplexer/demultiplexer 1208. In the phase modulator 1218, an electrostrictive vibrator is used to modulate the with 6 kHz. The phase modulator 1218 performs periodic adjustment to a condition for causing interference using the optical path length as the parameter. Fourth, a fixed reflecting mirror 1219 is used in place of an optical system 1205 including a collimator lens and a movable reflecting mirror, and a second Michelson optical circuit 1220 is arranged between an optical detector 1202 and a second optical waveguide 1210 to adjust the optical path length.
As opposed to this second Michelson optical circuit 1220, the optical multiplexer/demultiplexer 1208, a second portion 1209b of the first optical waveguide 1209, the second optical waveguide 1210, a third optical waveguide 1211, the fourth optical waveguide 1213, the terminal 1207 connecting the test sample, and the fixed reflecting mirror 1219 constitute the first Michelson optical circuit. A light beam propagates in the optical fiber in the first Michelson optical circuit, while a light beam propagates in the air in the second Michelson optical circuit. A polarized light beam emerging from the second optical waveguide 1210 is demultiplexed into two light beams by a half mirror 1224 through a collimator lens 1222 and a polarizing beam splitter 1223. A fixing reflecting mirror 1225 reflects one light beam, while a movable reflecting mirror 1226 reflects the other light beam. The reflected light beams are coupled by the half mirror 1224. The resultant light beam passes through a collimator lens 1227 and is incident on the optical detector 1202. The movable reflecting mirror 1226 changes the optical path length to cause interference between a reference light beam reflected by the fixed reflecting mirror 1219 and the light beam reflected by the test sample 1206. Reference numeral 1228a denotes an input terminal of the second portion 1209b of the first optical waveguide 1209; and 1228b, an output terminal of the second optical waveguide 1210.
As described above, in the return loss measuring device using the Michelson interference circuits and the superluminescent diode (SLD) as the light source in accordance with the OLCR method using as the basic arrangement the interference circuit using the low coherent light source, a high measurement sensitivity of 60 dB or more can be obtained at a spatial resolution of 100 xcexcm or less.
The SLD used in the above OLCR method is very expensive, and the polarization maintaining optical fibers used in the entire first Michelson optical circuit and the polarization controller for aligning the plane of polarization of a light beam emitted by the light source with the polarization maintaining plane of the polarization maintaining optical fiber are generally expensive.
Improvements have been made for low-cost return loss measuring devices. xe2x80x9cStudy of coherent return loss measurement using optical fiber delay linexe2x80x9d (Technical Report of the Institute of Electronic, Information, and Communication Engineers of Japan: EMD92-40, Masaru Kobayashi et. al., August, 1992) describes a method of obtaining an OLCR arrangement using a Fabry-Pxc3xa9rot semiconductor laser.
The arrangement of this OLCR method is shown in FIG. 25. The arrangement in FIG. 25 is based on the arrangement shown in FIG. 22, and only differences will be described below. First, a Fabry-Pxc3xa9rot semiconductor laser is used as a light source 1201. Although this semiconductor laser has a high degree of polarization, a large number of longitudinal modes are excited at an interval of 1.11 nm in the 1.3-xcexcm band to obtain an apparent spectral profile, thereby obtaining low coherency but a high degree of polarization. The OLCR arrangement is obtained using this. Second, a fixed reflecting mirror 1219 is used in place of an optical system 1205 including a lens 1203 and a movable mirror 1204, and an optical fiber delay line 1229 inserted midway along a third optical waveguide 1211 adjusts the optical path length. This optical fiber delay line 1229 stretches/contracts the reference optical fiber on a fine movement stage. Any one of the longitudinal modes is adjusted to satisfy the interference condition by this stretching/contraction.
In light beam propagation using the light source having a high degree of polarization like a semiconductor laser while maintaining the state of polarization, the plane of polarization of a light beam emitted by the semiconductor laser is converted into the polarization maintaining direction of the polarization maintaining optical fiber using a means for arbitrarily changing the state of polarization like a polarization controller. The converted light beam must be guided to the polarization maintaining optical fiber. The semiconductor laser is used in place of the SLD, and the arrangement cost is reduced accordingly. However, the number of polarization maintaining optical fibers is large, and the polarization controller is used. The return loss measuring device of the above method is still expensive.
The means for aligning the state of polarization with the polarization maintaining plane of the polarization maintaining optical fiber is generally expensive and complex. It is also possible to incorporate the polarization maintaining optical fiber as a semiconductor laser module while aligned with the plane of polarization of the exist light beam from the laser in advance. This semiconductor laser module is expensive. Much care must be taken for use of the semiconductor laser incorporating this semiconductor laser module in a measuring device requiring high precision. In particular, in the return loss measuring device for measuring a reflected light beam of 60 dB or more using interference in the Michelson optical system, high-precision adjustment is required to align the plane of polarization of a light beam with the polarization maintaining plane.
It is, therefore, the principal object of the present invention to provide a method and apparatus for maintaining an incident optical signal having a low degree of polarization in a specific state of polarization at an output terminal of an optical waveguide without positively using a special state-of-polarization control means.
It is another object of the present invention to provide a more inexpensive, compact, easy-to-handle light beam return loss measuring device than a conventional one.
In order to achieve the above objects of the present invention, there is provided a method of maintaining an incident optical signal having a low degree of polarization in a specific state of polarization at an output terminal of an optical waveguide, comprising the steps of inputting the optical signal having a low degree of polarization to an input section having as an input terminal an optical waveguide incapable of maintaining a state of polarization, and outputting an optical signal having the specific state of polarization to an output terminal of an output section through the output section having, in at least part thereof, an optical waveguide capable of maintaining a state of polarization and having no branches.
A conventional system having a light source having a high degree of polarization and an optical waveguide system capable of maintaining the state of polarization must be axially aligned with high precision. A state-of-polarization control means is inserted and adjusted to align the optical axis after polarization with the optical axis of the optical waveguide. An axial error caused by perturbation makes the state of polarization unstable to draw an irregular, unstable locus on the Poincarxc3xa9 sphere. Stability cannot be obtained unless the state-of-polarization control means is adjusted again to perform axial alignment. The state-of-polarization control means can align the plane of polarization, but cannot realize an arbitrary state of polarization.
A system having a light source having a low degree of polarization, an optical waveguide incapable of maintaining a state of polarization, and an optical waveguide capable of maintaining a state of polarization basically requires no state-of-polarization control means. The interaction between the light source having a low degree of polarization and the optical waveguide capable of maintaining the state of polarization allows stabilizing the state of polarization at the output terminal of the optical waveguide capable of maintaining the state of polarization.
Inserting the state-of-polarization control means allows controlling the state of polarization on the longitude of the Poincarxc3xa9 sphere. A change in position on the longitude and the stop at this position can stably maintain the corresponding state of polarization. Operation of controlling the state of polarization and stably maintaining it is conventionally permitted on only the equator (linear polarization), but is allowed in other regions for the first time according to the present invention. The degree of freedom of realizing various states of polarization increases, and a variety of possibilities for advancement can be brought about in research and development.